As spartaque12 already pointed out it's caused by occluders which are invisible meshes used for occlusion culling. The renderer skips rendering of any objects which are covered by one (or more) of these occluders from the current camera viewpoint.

To fix the issue you should ideally delete the offending occluder. Unfortunately I'm not aware of any tools that visualize occluders which can make it difficult to identify which of them need to be deleted; MWE's Jump To function might help a bit with that. In this particular case it looks like you're modifying the tutorial mission which only has a total of two occluders with occluder2 being the one you need to get rid of from what I can tell.